1. Field of the Invention
The present invention relates to an image processing apparatus provided with an image sensor comprising an effective pixel portion and a shaded pixel portion, and more particularly, it relates to a new technique to set the black level of the image sensor so as to improve image processing accuracy.
2. Description of the Related Art
In general, in image sensing devices, in order to suppress black floating and black level depression of a video signal (picked-up image) of an object to be sensed or picked up, there is required an image processing apparatus that always serves to correct the black level of the picked-up image.
For instance, in conventional image processing apparatuses, it is necessary to subtract a noise level due to a dark current (resulting from thermal noise of semiconductors) from the video signal so as to make the black level of the video signal constant (suppress black floating and black level depression). Accordingly, the video signal is corrected by referring to an output signal (hereinafter referred to as an “OB signal”) from shaded pixels (hereinafter abbreviated as “OB”) called an optical black to detect the dark current component.
In the above-mentioned prior art, there are provided, in a solid-state image sensor, an image pickup region formed of pixels to be photoelectrically converted and a shaded region (hereinafter referred to as an “OB portion”) formed by shading pixels of the same structure as that of the image pickup region, and signal output portions from the respective regions are dc clamped so as to stabilize the black level.
As a result, even if the dark current is increased due to a temperature rise, a signal of the OB portion output from the solid-state image sensor is dc clamped through an OB clamp circuit to a reference voltage of the circuit, whereby the black level is held constant.
In addition, the OB clamp circuit is generally arranged at a preceding or upstream stage of a gain control circuit, so it is constructed such that a black level variation due to a gain change can be suppressed to a minimum.
In the conventional OB clamp circuit, however, the error of the clamp circuit is amplified at the time when the gain is increased, so the noise level can not be removed to a satisfactory extent, thus resulting in video images with a poor S/N (signal to noise) ratio.
Moreover, in order to accurately correct the black level when the gain is increased, it is necessary to correct the black level again after the increase of the gain has been completed, thus inviting a large increase in size and cost of the circuit.
Further, in the above-mentioned prior art, it is constructed such that the black floating and black level depression of the video signal (picked-up image) can always be suppressed accurately by focusing attention on the OB portion alone without considering the brightness of an external world (picture to be measured or imaged), so it is very difficult to perform correction while following the external world that is constantly changing.
Accordingly, in order to deal with the above-mentioned various problems, there has been proposed an image sensing device that includes an OB clamp circuit and an offset addition circuit, and serves to correct the black level by selecting an amount of offset corresponding to an ambient temperature based on a correction table stored in a CPU (see, for instance, a first patent document: Japanese patent application laid-open No. 2004-080168).
Also, another image sensing device has been proposed which includes an effective region into which incident light comes, a shading region which shades incident light, and a region with no light receiving element, and which corrects the black level by using parameters such as an exposure time, temperature data, etc., (see, for example, a second patent document: Japanese patent application laid-open No. 2004-120492). In this apparatus, the data of the region with no light receiving element is used for the purpose of correction.
In prior art image processing apparatuses such as, for example, those as described in the above-mentioned first and second patent documents, there is a problem that it is difficult to completely remove the noise level, for example, in case where the gain of a signal processing section is very large, thus making it impossible to achieve image processing with high precision.
In addition, in either of the conventional apparatuses described in the above-mentioned first and second patent documents, there is also another problem that it is necessary to provide a control section and a temperature sensor for controlling the black level in an accurate manner, thus resulting in an increase in the circuit size.